The present invention relates to a process for increasing the resistance to water of sorbent layers for chromatography which have been applied to supports.
Layer chromatography is becoming ever more widespread both as a rapid micro- or ultramicro-analytical method and, using thicker layers of sorbent, in preparative chemistry. The sorbents customarily used, such as silica gel, kieselguhr, aluminum oxides, magnesium silicates or surface-modified sorbents, must display an adequate adhesion to the supports under the action of mechanical stress and/or the solvents customarily used in chromatography. In order to achieve this, diverse adhesives or binders have already been proposed, for example gypsum, starch, carboxymethylcellulose or polyvinyl alcohol.
However, these known binders have the disadvantage that they either effect adequate adhesion only in very high concentration or that, when subjected to the action of corrosive reagents and, in some cases, subsequent heating, this being a procedure which is very frequently required in layer chromatography for the detection of organic substances, they become darkly colored making detection more difficult or even impossible.
In German Pat. No. 1,442,446 polyvinyl compounds containing carboxyl groups, and the salts of these compounds, which do not have these disadvantages, are described as binders. The alkali metal and alkaline earth metal salts and also magnesium salts of polyacrylic and polymethacrylic acids are mentioned as being particularly suitable; by reason of economic considerations, the sodium salts of these acids are preferred. When added to the conventional sorbents in an amount of 0.1--10% by weight, these binders give high resistance to abrasion and have good resistance to the corrosive reagents used in the detection procedures mentioned.
The polymeric binders containing carboxyl groups are employed, above all, in the form of their salts and not with free COOH groups because, in the form of salts, they have the highest viscosity in aqueous systems. They thus permit the preparation of stable suspensions of the sorbents, which suspensions do not tend to sediment during processing. Coating of supports with aqueous sorbent suspensions which contain the free polyacrylic or polymethacrylic acids as the binders is, on the other hand, difficult with respect to the preparation techniques required and results in unsatisfactory precoated preparations. The low viscosity of these suspensions makes the production by mechanical means of pre-coated preparations, for chromatography, more difficult since the sorbent already settles out as a sediment during the processing stage.
However, the layers comprising the polymeric binders containing carboxyl groups in the form of their salts have proved to possess a disadvantage, viz., a deficient resistance to water. This deficiency can be ascribed to the fact that the salts of polyacrylic and polymethacrylic acids swell extensively and dissolve in the presence of water. This, in turn, has the effect that a layer which adhered firmly beforehand, dissolves off the particular support under the action of water. However, a number of separation problems in chromatography necessitate the use of water or mixtures of water with other solvents as the solvent system. In order to enable such separations to be carried out, water-resistant sorbent layers are required, so that the development of the chromatogram is not made more difficult or even impossible by the dissolution of the layer from the particular support, which would otherwise occur. Furthermore, in sorbent layers which are not water-resistant, the binders can be partially dissolved out of the layer by the action of solvent systems which are purely aqueous or which contain water. This has a particularly adverse effect on the analytical detection of substances with R.sub.f values of more than 0.7. Moreover, in preparative layer chromatography, this effect makes it more difficult to isolate the pure substances with R.sub.f values of more than 0.7 which have been separated by the chromatography.
In order to solve specific separation problems, the sorbent layers, in layer chromatography, are frequently impregnated prior to the actual chromatography. Since the impregnating liquids are frequently in the form of purely aqueous solutions, resistance of the sorbent layers to water is required in this case also.
Water-resistant pre-coated preparations for chromatography are also necessary when, after chromatography has been carried out, detection reagents are employed in the form of their aqueous solutions to detect the individual substances which have been separated.
Furthermore, German Offenlegungsschrift No. 1,698,244 describes dispersions of crosslinkable or self-crosslinking synthetic resins or their precursors as binders for chromatographic sorbent layers. There are mentioned acrylate and methacrylate copolymers, which by copolymerization of monomers with reactive groups can also enter into subsequent reactions, as being particularly advantageous. However, in this case, due to the introduction of the additional constituents, there is an increased danger that if the reaction is not complete, very diverse substances will remain in the sorbent layer; these can be dissolved out of the layer by the action of solvent systems used in chromatography and, thus, interfere in the evaluation of the chromatogram. When polymers containing ester groups are used as binders, a hydrophobic constituent is introduced into the sorbent layer, making it more difficult to wet the layer with water. Furthermore, when sorbent suspensions which contain these ester group-containing polymers as binders are processed, their low viscosity in aqueous systems gives rise to difficulties similar to those encountered when free poly(meth)-acrylic acids are used as binders.